<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>index04</title>
    <style>
         
        p{
            background-color: red;
            height: 30px;
        }

        p:empty{
            background-color: green;
        }

        p:first-child{
            color: yellow;
        }

        :not(a){
            color: red;
        }
        p:not(:first-child){
            color: pink;
        }

        input:focus{
            background-color: yellow;
        }

        input:valid{
            background-color: rgb(214, 221, 214);
        }

        input:checked{
            background-color: yellow;
            width: 20px;
            height: 20px;
        }

    </style>
</head>
<body>
    <a href="https://www.baidu.com">hello world</a>

    <div>
        <p>hello world</p>
        <p>hello world</p>
    </div>
    <div>
        <p></p>
        <p>hello world</p>
    </div>

    <input type="text" placeholder="请输入内容">

    <form>
        <input type="checkbox" placeholder="请输入内容">
        <input type="checkbox" placeholder="请输入内容">
        <input type="checkbox" placeholder="请输入内容">
    </form>


</body>
</html>